Because gallium nitride, the 3rd generation wide-band gap semiconductor material, has properties of wide band gap, high saturated electron drift velocity, high critical electric field and high thermal conductivity, it is more suitable for making high-temperature, high-frequency, high-pressure and high-power power switching devices than silicon and gallium arsenide. GaN devices have a good application prospect in a high-frequency and high-power microwave devices. Since 1990s, the development of GaN devices has been a research focus of power electronic devices. Due to the lack of GaN native substrates, GaN devices are grown on heterogeneous substrates, such as sapphire, silicon carbide or silicon. Among these substrates, silicon substrate has the largest size and the lowest cost. Therefore, the GaN materials and GaN devices grown on a silicon substrate attracts extensive attention.
However, it is difficult to grown GaN epitaxial layers on a silicon substrate due to huge lattice mismatch and thermal mismatch between the silicon substrate and the epitaxial nitride layers. Thus, an AlN nucleation layer is generally required to be grown first to prevent the melt back etching of the silicon substrate by gallium atoms, especially in the ammonia ambient. In addition, it is difficult to obtain a homogeneous and continuous GaN epitaxial layer, since GaN has a poor wetting property on the silicon substrate. A high-quality aluminum nitride nucleation layer is essential when growing a GaN device on a silicon substrate. With the aid of the nitride nucleation layer, a homogeneous and continuous GaN epitaxial layer with a smooth surface can be grown on the silicon substrate. However, because of the large thermal mismatch between GaN and Si, a huge tensile stress is generated during cooling down to the room temperature after the GaN layer is grown at a high temperature. When the GaN epitaxial layer thickness exceeds a critical value (e.g., 1 μm), the epitaxial layer may crack. To avoid the crack generation, a compressive stress may be introduced to the epitaxial structures at high temperature. If the average value of the compressive stress is equal to that of the tensile stress, the warpage of the silicon substrate can be minimized.
The method of introducing compensatory compressive stress at a high temperature is referred as stress engineering. The stress engineering usually introduces a stress according to the lattice mismatch between epitaxial layers and the substrates. If the lattice constant of the substrate is less than that of the epitaxial layer, a compressive stress may be introduced; otherwise, if the lattice constant of the substrate is greater than that of the epitaxial layer, a tensile stress may be introduced. The conventional method may include: forming a low-temperature AlN interlayer, which may refer to the article published on “Appl. Phys. Lett. Volume 80, Issue 20” issued in 2002 by A. Dadgar et al., or the article published on “Japanese Journal of Applied Physics, Volume 37, Issue 12B, pp. L1540-L1542” issued in 1998 by Hiroshi Amano et al.; forming an AlN/AlGaN superlattice structure, which may refer to the article published on “Appl. Phys. Lett. v81, p 604” issued in 2002 by Hong-Mei Wang et al.; or forming a GaN/AlGaN superlattice structure, which may refer to the article published on “Appl. Phys. Lett. v75, p 2073” issued in 1999 by S. A. Nikishin et al.; or forming an AlN/GaN superlattice structure, which may refer to the article published on “Appl. Phys. Lett. v79, p 3230” issued in 2001 by Eric Feltin et al.
The low-temperature AlN interlayer is initially used to grow a crack-free AlGaN epitaxial layer on a GaN template. Because the lattice constant of AlGaN is less than that of GaN, the growth stress in AlGaN is thus tensile stress. When the AlGaN epitaxial layer has a thickness greater than the critical value, the epitaxial layer may introduce cracks and no device can be fabricated then. By introducing the low-temperature AlN interlayer, the tensile stress may be compensated effectively and the crack generation can be avoided. Afterwards, this method is used to grown crack-free GaN epitaxial layers on silicon substrates. However, the low-temperature AlN interlayer may have influence on the crystalline quality of the GaN epi-layers. It has been found that the quality of the GaN epitaxial layer may be degraded greatly after the introduction of the low-temperature AlN interlayer. Edge type dislocations are generated at the interface between the low-temperature AlN interlayer and the GaN epitaxial layer, which increases the overall dislocation density. Furthermore, unexpected particles may be generated due to temperature ramping, which may decrease production yield.